物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 11. GEOGRAPHICAL DISTRIBUTION. Page 2
MEANS OF DISPERSAL.
Sir C. Lyell and other authors have ably treated this subject. I cangive here only the briefest abstract of the more important facts.Change of climate must have had a powerful influence on migration: aregion when its climate was different may have been a high road formigration, but now be impassable; I shall, however, presently have todiscuss this branch of the subject in some detail. Changes of level inthe land must also have been highly influential: a narrow isthmus nowseparates two marine faunas; submerge it, or let it formerly have beensubmerged, and the two faunas will now blend or may formerly haveblended: where the sea now extends, land may at a former period haveconnected islands or possibly even continents together, and thus haveallowed terrestrial productions to pass from one to the other. Nogeologist will dispute that great mutations of level have occurredwithin the period of existing organisms. Edward Forbes insisted thatall the islands in the Atlantic must recently have been connected withEurope or Africa, and Europe likewise with America. Other authors havethus hypothetically bridged over every ocean, and have united almostevery island to some mainland. If indeed the arguments used by Forbesare to be trusted, it must be admitted that scarcely a single islandexists which has not recently been united to some continent. This viewcuts the Gordian knot of the dispersal of the same species to the mostdistant points, and removes many a difficulty: but to the best of myjudgment we are not authorized in admitting such enormous geographicalchanges within the period of existing species. It seems to me that wehave abundant evidence of great oscillations of level in ourcontinents; but not of such vast changes in their position andextension, as to have united them within the recent period to eachother and to the several intervening oceanic islands. I freely admitthe former existence of many islands, now buried beneath the sea,which may have served as halting places for plants and for manyanimals during their migration. In the coral-producing oceans suchsunken islands are now marked, as I believe, by rings of coral oratolls standing over them. Whenever it is fully admitted, as I believeit will some day be, that each species has proceeded from a singlebirthplace, and when in the course of time we know something definiteabout the means of distribution, we shall be enabled to speculate withsecurity on the former extension of the land. But I do not believethat it will ever be proved that within the recent period continentswhich are now quite separate, have been continuously, or almostcontinuously, united with each other, and with the many existingoceanic islands. Several facts in distribution,--such as the greatdifference in the marine faunas on the opposite sides of almost everycontinent,--the close relation of the tertiary inhabitants of severallands and even seas to their present inhabitants,--a certain degree ofrelation (as we shall hereafter see) between the distribution ofmammals and the depth of the sea,--these and other such facts seem tome opposed to the admission of such prodigious geographicalrevolutions within the recent period, as are necessitated on the viewadvanced by Forbes and admitted by his many followers. The nature andrelative proportions of the inhabitants of oceanic islands likewiseseem to me opposed to the belief of their former continuity withcontinents. Nor does their almost universally volcanic compositionfavour the admission that they are the wrecks of sunkencontinents;--if they had originally existed as mountain-ranges on theland, some at least of the islands would have been formed, like othermountain-summits, of granite, metamorphic schists, old fossiliferousor other such rocks, instead of consisting of mere piles of volcanicmatter.
I must now say a few words on what are called accidental means, butwhich more properly might be called occasional means of distribution.I shall here confine myself to plants. In botanical works, this orthat plant is stated to be ill adapted for wide dissemination; but fortransport across the sea, the greater or less facilities may be saidto be almost wholly unknown. Until I tried, with Mr. Berkeley's aid, afew experiments, it was not even known how far seeds could resist theinjurious action of sea-water. To my surprise I found that out of 87kinds, 64 germinated after an immersion of 28 days, and a few survivedan immersion of 137 days. For convenience sake I chiefly tried smallseeds, without the capsule or fruit; and as all of these sank in a fewdays, they could not be floated across wide spaces of the sea, whetheror not they were injured by the salt-water. Afterwards I tried somelarger fruits, capsules, etc., and some of these floated for a longtime. It is well known what a difference there is in the buoyancy ofgreen and seasoned timber; and it occurred to me that floods mightwash down plants or branches, and that these might be dried on thebanks, and then by a fresh rise in the stream be washed into the sea.Hence I was led to dry stems and branches of 94 plants with ripefruit, and to place them on sea water. The majority sank quickly, butsome which whilst green floated for a very short time, when driedfloated much longer; for instance, ripe hazel-nuts sank immediately,but when dried, they floated for 90 days and afterwards when plantedthey germinated; an asparagus plant with ripe berries floated for 23days, when dried it floated for 85 days, and the seeds afterwardsgerminated: the ripe seeds of Helosciadium sank in two days, whendried they floated for above 90 days, and afterwards germinated.Altogether out of the 94 dried plants, 18 floated for above 28 days,and some of the 18 floated for a very much longer period. So that as64/87 seeds germinated after an immersion of 28 days; and as 18/94plants with ripe fruit (but not all the same species as in theforegoing experiment) floated, after being dried, for above 28 days,as far as we may infer anything from these scanty facts, we mayconclude that the seeds of 14/100 plants of any country might befloated by sea-currents during 28 days, and would retain their powerof germination. In Johnston's Physical Atlas, the average rate of theseveral Atlantic currents is 33 miles per diem (some currents runningat the rate of 60 miles per diem); on this average, the seeds of14/100 plants belonging to one country might be floated across 924miles of sea to another country; and when stranded, if blown to afavourable spot by an inland gale, they would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but ina much better manner, for he placed the seeds in a box in the actualsea, so that they were alternately wet and exposed to the air likereally floating plants. He tried 98 seeds, mostly different from mine;but he chose many large fruits and likewise seeds from plants whichlive near the sea; and this would have favoured the average length oftheir flotation and of their resistance to the injurious action of thesalt-water. On the other hand he did not previously dry the plants orbranches with the fruit; and this, as we have seen, would have causedsome of them to have floated much longer. The result was that 18/98 ofhis seeds floated for 42 days, and were then capable of germination.But I do not doubt that plants exposed to the waves would float for aless time than those protected from violent movement as in ourexperiments. Therefore it would perhaps be safer to assume that theseeds of about 10/100 plants of a flora, after having been dried,could be floated across a space of sea 900 miles in width, and wouldthen germinate. The fact of the larger fruits often floating longerthan the small, is interesting; as plants with large seeds or fruitcould hardly be transported by any other means; and Alph. de Candollehas shown that such plants generally have restricted ranges.
But seeds may be occasionally transported in another manner. Drifttimber is thrown up on most islands, even on those in the midst of thewidest oceans; and the natives of the coral-islands in the Pacific,procure stones for their tools, solely from the roots of driftedtrees, these stones being a valuable royal tax. I find on examination,that when irregularly shaped stones are embedded in the roots oftrees, small parcels of earth are very frequently enclosed in theirinterstices and behind them,--so perfectly that not a particle couldbe washed away in the longest transport: out of one small portion ofearth thus COMPLETELY enclosed by wood in an oak about 50 years old,three dicotyledonous plants germinated: I am certain of the accuracyof this observation. Again, I can show that the carcasses of birds,when floating on the sea, sometimes escape being immediately devoured;and seeds of many kinds in the crops of floating birds long retaintheir vitality: peas and vetches, for instance, are killed by even afew days' immersion in sea-water; but some taken out of the crop of apigeon, which had floated on artificial salt-water for 30 days, to mysurprise nearly all germinated.
large as theseed of a vetch. Thus seeds might occasionally be transported to greatdistances; for many facts could be given showing that soil almosteverywhere is charged with seeds. Reflect for a moment on the millionsof quails which annually cross the Mediterranean; and can we doubtthat the earth adhering.
Living birds can hardly fail to be highly effective agents in thetransportation of seeds. I could give many facts showing howfrequently birds of many kinds are blown by gales to vast distancesacross the ocean. We may I think safely assume that under suchcircumstances their rate of flight would often be 35 miles an hour;and some authors have given a far higher estimate. I have never seenan instance of nutritious seeds passing through the intestines of abird; but hard seeds of fruit will pass uninjured through even thedigestive organs of a turkey. In the course of two months, I picked upin my garden 12 kinds of seeds, out of the excrement of small birds,and these seemed perfect, and some of them, which I tried, germinated.But the following fact is more important: the crops of birds do notsecrete gastric juice, and do not in the least injure, as I know bytrial, the germination of seeds; now after a bird has found anddevoured a large supply of food, it is positively asserted that allthe grains do not pass into the gizzard for 12 or even 18 hours. Abird in this interval might easily be blown to the distance of 500miles, and hawks are known to look out for tired birds, and thecontents of their torn crops might thus readily get scattered. Mr.Brent informs me that a friend of his had to give up flyingcarrier-pigeons from France to England, as the hawks on the Englishcoast destroyed so many on their arrival. Some hawks and owls bolttheir prey whole, and after an interval of from twelve to twentyhours, disgorge pellets, which, as I know from experiments made in theZoological Gardens, include seeds capable of germination. Some seedsof the oat, wheat, millet, canary, hemp, clover, and beet germinatedafter having been from twelve to twenty-one hours in the stomachs ofdifferent birds of prey; and two seeds of beet grew after having beenthus retained for two days and fourteen hours. Freshwater fish, Ifind, eat seeds of many land and water plants: fish are frequentlydevoured by birds, and thus the seeds might be transported from placeto place. I forced many kinds of seeds into the stomachs of dead fish,and then gave their bodies to fishing-eagles, storks, and pelicans;these birds after an interval of many hours, either rejected the seedsin pellets or passed them in their excrement; and several of theseseeds retained their power of germination. Certain seeds, however,were always killed by this process.
Although the beaks and feet of birds are generally quite clean, I canshow that earth sometimes adheres to them: in one instance I removedtwenty-two grains of dry argillaceous earth from one foot of apartridge, and in this earth there was a pebble quite as large as theseed of a vetch. Thus seeds might occasionally be transported to greatdistances; for many facts could be given showing that soil almosteverywhere is charged with seeds. Reflect for a moment on the millionsof quails which annually cross the Mediterranean; and can we doubtthat the earth adhering to their feet would sometimes include a fewminute seeds? But I shall presently have to recur to this subject.
As icebergs are known to be sometimes loaded with earth and stones,and have even carried brushwood, bones, and the nest of a land-bird, Ican hardly doubt that they must occasionally have transported seedsfrom one part to another of the arctic and antarctic regions, assuggested by Lyell; and during the Glacial period from one part of thenow temperate regions to another. In the Azores, from the large numberof the species of plants common to Europe, in comparison with theplants of other oceanic islands nearer to the mainland, and (asremarked by Mr. H. C. Watson) from the somewhat northern character ofthe flora in comparison with the latitude, I suspected that theseislands had been partly stocked by ice-borne seeds, during the Glacialepoch. At my request Sir C. Lyell wrote to M. Hartung to inquirewhether he had observed erratic boulders on these islands, and heanswered that he had found large fragments of granite and other rocks,which do not occur in the archipelago. Hence we may safely infer thaticebergs formerly landed their rocky burthens on the shores of thesemid-ocean islands, and it is at least possible that they may havebrought thither the seeds of northern plants.
Considering that the several above means of transport, and thatseveral other means, which without doubt remain to be discovered, havebeen in action year after year, for centuries and tens of thousands ofyears, it would I think be a marvellous fact if many plants had notthus become widely transported. These means of transport are sometimescalled accidental, but this is not strictly correct: the currents ofthe sea are not accidental, nor is the direction of prevalent gales ofwind. It should be observed that scarcely any means of transport wouldcarry seeds for very great distances; for seeds do not retain theirvitality when exposed for a great length of time to the action ofseawater; nor could they be long carried in the crops or intestines ofbirds. These means, however, would suffice for occasional transportacross tracts of sea some hundred miles in breadth, or from island toisland, or from a continent to a neighbouring island, but not from onedistant continent to another. The floras of distant continents wouldnot by such means become mingled in any great degree; but would remainas distinct as we now see them to be. The currents, from their course,would never bring seeds from North America to Britain, though theymight and do bring seeds from the West Indies to our western shores,where, if not killed by so long an immersion in salt-water, they couldnot endure our climate. Almost every year, one or two land-birds areblown across the whole Atlantic Ocean, from North America to thewestern shores of Ireland and England; but seeds could be transportedby these wanderers only by one means, namely, in dirt sticking totheir feet, which is in itself a rare accident. Even in this case, howsmall would the chance be of a seed falling on favourable soil, andcoming to maturity! But it would be a great error to argue thatbecause a well-stocked island, like Great Britain, has not, as far asis known (and it would be very difficult to prove this), receivedwithin the last few centuries, through occasional means of transport,immigrants from Europe or any other continent, that a poorly-stockedisland, though standing more remote from the mainland, would notreceive colonists by similar means. I do not doubt that out of twentyseeds or animals transported to an island, even if far lesswell-stocked than Britain, scarcely more than one would be so wellfitted to its new home, as to become naturalised. But this, as itseems to me, is no valid argument against what would be effected byoccasional means of transport, during the long lapse of geologicaltime, whilst an island was being upheaved and formed, and before ithad become fully stocked with inhabitants. On almost bare land, withfew or no destructive insects or birds living there, nearly everyseed, which chanced to arrive, would be sure to germinate and survive.
DISPERSAL DURING THE GLACIAL PERIOD.
The identity of many plants and animals, on mountain-summits,separated from each other by hundreds of miles of lowlands, where theAlpine species could not possibly exist, is one of the most strikingcases known of the same species living at distant points, without theapparent possibility of their having migrated from one to the other.It is indeed a remarkable fact to see so many of the same plantsliving on the snowy regions of the Alps or Pyrenees, and in theextreme northern parts of Europe; but it is far more remarkable, thatthe plants on the White Mountains, in the United States of America,are all the same with those of Labrador, and nearly all the same, aswe hear from Asa Gray, with those on the loftiest mountains of Europe.Even as long ago as 1747, such facts led Gmelin to conclude that thesame species must have been independently created at several distinctpoints; and we might have remained in this same belief, had notAgassiz and others called vivid attention to the Glacial period,which, as we shall immediately see, affords a simple explanation ofthese facts. We have evidence of almost every conceivable kind,organic and inorganic, that within a very recent geological period,central Europe and North America suffered under an Arctic climate. Theruins of a house burnt by fire do not tell their tale more plainly,than do the mountains of Scotland and Wales, with their scored flanks,polished surfaces, and perched boulders, of the icy streams with whichtheir valleys were lately filled. So greatly has the climate of Europechanged, that in Northern Italy, gigantic moraines, left by oldglaciers, are now clothed by the vine and maize. Throughout a largepart of the United States, erratic boulders, and rocks scored bydrifted icebergs and coast-ice, plainly reveal a former cold period.
The former influence of the glacial climate on the distribution of theinhabitants of Europe, as explained with remarkable clearness byEdward Forbes, is substantially as follows. But we shall follow thechanges more readily, by supposing a new glacial period to come slowlyon, and then pass away, as formerly occurred. As the cold came on, andas each more southern zone became fitted for arctic beings andill-fitted for their former more temperate inhabitants, the latterwould be supplanted and arctic productions would take their places.The inhabitants of the more temperate regions would at the same timetravel southward, unless they were stopped by barriers, in which casethey would perish. The mountains would become covered with snow andice, and their former Alpine inhabitants would descend to the plains.By the time that the cold had reached its maximum, we should have auniform arctic fauna and flora, covering the central parts of Europe,as far south as the Alps and Pyrenees, and even stretching into Spain.The now temperate regions of the United States would likewise becovered by arctic plants and animals, and these would be nearly thesame with those of Europe; for the present circumpolar inhabitants,which we suppose to have everywhere travelled southward, areremarkably uniform round the world. We may suppose that the Glacialperiod came on a little earlier or later in North America than inEurope, so will the southern migration there have been a littleearlier or later; but this will make no difference in the finalresult.